Establishment of harmonic frequencies in a penning-type discharge



Sept. 8, 1970 E. D. SHIPLEY ETAL 3,528,034

' ESTABLISHMENT OF HARMONIC FREQUENCIES IN A PENNING-TYPE DISCHARGE Filed July 26, 1968 3 Sheets-Sheet l 3 Es VACUUM Fig.1 H

-FREQUENCY VOLTAGE ANALYZER SUPPLY 4 PRIOR ART AMPLITUDE FREQUENCY PRIOR ART f INVENTORS. Elwood D. Shipley BY Oliyer C. Yonfs m4 4 4- ATTORNEY.

E. o. SHIPLEY ETAL 3,528,034 ESTABLISHMENT OF HARMONIC FREQUENCIES IN A PENNING-TYPE DISCHARGE Filed July 26, 1968 3 Sheets-Sheet 2 Sept. 8, 1970 FREQUENCY vo-mea ANALYZER SUPPLY Fig 3 LLI O D 3 2 l I I I i I l I FREQUENCY Fig.4

INVENTORS.

Elwood D. Ship/ey BY Oliver C. Yonfs v p 7 E. D. SHIPLEY ETAL 3,528,034 ESTABLISHMENT OF HARMONIC FREQUENCIES IN A PENNING-TYPE DISCHARGE Filed July 26, 1968 3 Sheets-Sheet 3 fi V7 TO VACUUM H 3 )b FREQUENCY ANALYZER 1G VOLTAGE SUPPLY INVENTOR. Elwood D. $h/p/ey BY Oliver C. Yonts ATTORNEY United States Patent Office 3,528,034 Patented Sept. 8, 1970 3,528,034 ESTABLIS l NT OF HARMONIC FREQUENCIES IN A PENNING-TYPE DISCHARGE Elwood D. Shipley, Knoxville, and Oliver C. Yonts, Oak

Ridge, Tenn, assignors to the United States of America as represented by the United States Atomic Energy Commission Filed July 26, 1968, Ser. No. 747,953 Int. Cl. H03c 1/28; H01j 25/02 US. Cl. 332-5 4 Claims ABSTRACT OF THE DISCLOSURE One cathode of a Penning-type discharge device is divided in half with a resonant circuit joining the two parts. The plasma discharge therein, which otherwise contains a continuum of frequencies, is thereby modified so as to contain only harmonic frequencies, thereby substantially stabilizing the plasma. Such a device may also be used, if desired, as a simple frequency-modulated broadcasting unit by the application of an audio-range frequency to the voltage on the anode of the device.

BACKGROUND OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the US. Atomic Energy Commission.

The field of art to which the present invention pertains is in the production of stable plasmas wherein such plas mas are useful in the efiicient operation of controlled thermonuclear-type machines.

A typical prior art thermonuclear device is described in the patent to John S. Luce, Pat. No. 3,030,543, issued Apr. 17, 1962. In such a device, the plasma density that could be achieved was limited due to certain loss mechanisms, and the losses are sufficiently high to prevent exponentiation within the plasma. Most of these losses can be attributed to microinstabilities. These microinstabilities are believed to be associated with the radiofrequency signals that emanate from the plasma. Thus, there exists a need for some means for suppressing these microinstabilities such that the plasma density can be substantially increased in such prior art devices and in similar type devices such as will be described in the present invention hereinbelow.

SUMMARY OF THE INVENTION It is the object of the present invention to provide a means for substantially suppressing the microinstabilities associated with plasmas of plasma producing devices such that the density of such plasmas can be effectively increased and stabilized.

The above object has been accomplished in the present invention by modifying a conventional discharge device such that the frequency spectrum of the plasma thereof is changed from a continuum having a wide frequency distribution to a line spectra with the result that the plasma is substantially stabilized. The prior device, which includes a separated pair of parallel, circular plate cathodes with a ring anode spaced midway therebetween and a voltage supply connected between the anode and cathodes to establish a refluxing discharge between the cathodes, is modified in that one of the cathodes is divided along a diameter with one-half thereof connected to the grounded side of the voltage supply and the second half thereof connected to the first half through a parallel resonant circuit. With such a modification, the frequency spectrum of the discharge plasma is converted from a continuum to a line spectra resulting in a more stabilized plasma.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing of the basic circuit of a conventional Penning-type ion gage.

FIG. 2 is a plot illustrative of the frequency distribution obtained from a plasma within the device of FIG. 1.

FIG. 3 is a schematic drawing of a circuit of a modified device for producing harmonic frequencies in accordance with the above object of this invention.

FIG. 3a is another showing of the divided cathode of FIG. 3.

FIG. 4 is a plot illustrative of the frequency distribution obtained from the plasma of the device of FIG. 3.

FIG. 5 is a schematic diagram of a frequency modulation circuit as obtainable with a modification of the device of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The electrical circuit of a conventional prior art Penning-type discharge device is shown in FIG. 1. The device comprises a pair of parallel cathodes 1, 2 with a ring anode 3 spaced midway therebetween. These electrodes are encompassed within a vacuum chamber 11 which can be evacuated to a selected pressure by conventional means, not shown. A magnetic field of a selected value from 1000 to 2500 gauss and with lines perpendicular to the planes of the electrodes, as represented by the arrow H, is provided by permanent magnets or electromagnets, not shown. An appropriate potential for the electrodes is provided by a supply 4, and voltage and current are measured by meters 5, 6, respectively. Frequencies existing in the plasma within the device of FIG. 1 may be detected by an antenna or probe 7 which feeds a frequency analyzer 8. The unit 8 may be, for example, a spectrum analyzer made by Hewlett-Paclgard as their Model No. 8551A, or one made by Panoramic Electronics, Inc., as their Model No. SPA 3.

When the device of FIG. 1 is operated at a pressure of about 2X10 torr and with 2500 volts applied across the electrodes, the detected frequency spectrum from the plasma is similar to that shown in FIG. 2. That is, there is a continuum having a wide frequency distribution. It is these radiofrequency signals and their associated microinstabilities that limit the density of the plasma achievable within such prior art devices.

The present invention was conceived to overcome the above limitations of the prior art and FIG. 3 illustrates one embodiment in which this is accomplished. The device of FIG. 3 is substantially identical to the device of FIG. 1, and corresponding parts have the same reference numerals. In FIG. 3, the ring anode 3 is spaced midway between the cathode 2 and a cathode having separate halves 1a and 1b, as more clearly illustrated in FIG. 3a. As shown, cathode 2 and cathode half 1a are connected to one side of the voltage supply '4 which is also grounded, as shown. It should be understood that either side of the supply 4 could be grounded, as desired, and the operation of the device would be the same in either case. The cathode half 1b is electrically connected to the cathode half 1a through a parallel resonant circuit 9.

In the operation of the device of FIG. 3, using the conditions set forth above but with the resonant circuit 9 tuned to a selected value from 15 to 30 mHz. (depending on the value of magnetic field), a line spectra is obtained and is observed in the spectrum analyzer 8 with such a spectra as shown in FIG. 4. Thus, it should be evident that the prior art frequency spectrum as shown in FIG. 2 can be changed to that shown in FIG. 4 by the modifications as described above for FIG. 3. It should be noted that when the cathode half-plates 1a and 1b of FIG. 3 are shorted to each other the frequency spectrum returns to that shown in FIG. 2. The particular frequencies shown in FIG. 4 are the various harmonics of the frequency to which the resonant circuit 9 is tuned.

The following discussion gives a simplified explanation of the line spectrum as achieved with the device of FIG. 3. There are two components of RF field that can affect an electron circulating in a cycloidal fashion along the dotted line of FIG. 3a when the split cathode halves are driven by an external RF generator, for example. These are E" and EL where parallel and perpendicular refer to the magnetic field. We will consider only If the electron is near the slot between the two cathode halves when is a maximum, it will be moved radially inward or outward by some small amount each time it crosses the slot, thus changing its circulating frequency. An electron which is well away, from the slot when is a maximum or crosses the slot when is zero will not cycloid radially and will continue to circulate along its initial path. If there were many electrons distributed along the dotted circle, then an azimuthal bunching would take place until the remaining electrons were properly phased with respect to Replacing the external generator with the parallel resonant circuit shown in FIG. 3a does not change the picture since the induced voltage is a maximum when the electron bunch is directly opposite the cathode half. The electrons which move outward hit the anode ring of FIG. 3, and those which move inward continue to do so until their circulating frequency is twice f,,, It should be noted that f is the lowest frequency in the harmonic line spectrum. The influence of the fundamental frequency will just exactly cancel out and result in zero radial drift. The same argument can be extended to all circulating frequencies f =Kf where K=2, 3, It should be noted that the tuning is not critical, but f can be tuned by the external resonant circuit over a maximum of 30 percent. However, f is inversely proportional to the magnetic field and directly proportional to the electric field.

From the above discussion it should be self-evident that the harmonic line spectrum is produced by a resonance between the tuned split cathode and circulating bunches of electrons.

It should be noted that the same performance has been observed with variations of the circuit of FIG. 3. These variations include the use of a split anode ring 3 and connecting the resonant circuit 9 thereacross, and the use of a split anode cylinder in place of the split ring. Furthermore, similar operation is observed when additional electrons are introduced from a filament as well as when an axial discharge was produced.

An associated phenomenon with the continuum produced by FIG. 1 and the line spectrum produced by FIG. 3 is the DC. anode current. In general, the maximum DC. current is associated with the continuum and the minimum DC. current is associated with the line spectrum. Since the electrons are generated by secondary ionization processes, this implies an enhanced electron transport (easy removal) when the line spectrum is pres ent. It is this electron mobility that is useful in the operation of the device of FIG. 3 in reducing ion instabilities.

Since the electron bunch, as discussed above, is not sinusoidal in space, the RF spectrum will inherently be rich in harmonics of the circulating frequency. Such a device, therefore, gives rise to discrete frequencies which might be utilized for transmitting standardizing signals. In other uses, filters or tuned power amplifiers may be applied to the transmitted signal to remove certain of the frequencies and thereby produce a source of a few frequencies or of one frequency.

Still another use of the principles of the present invention is in frequency modulation as might be utilized in communication broadcasting. Such a use is illustrated in the embodiment as shown in FIG. 5 of the present invention. The device of FIG. 5 is similar to the circuit of FIG. 3 except a transformer 10 is utilized in the power supply lead whereby an A.C. signal may be impressed upon the anode ring 3. With a vacuum of less than 10- torr and with the resonant circuit 9 tuned to 30 mI-Iz definite frequency modulation was observed when an audiofrequency signal was applied by way of the transformer 10. The device of FIG. 5 has the advantages of being simple, stable in operation, and extremely rugged structurally. The device is essentially self-suflicient in the sense that a minimum of equipment is needed for control purposes, and even greater stability can be achieved, if desired, if a hot filament is utilized to supply extra electrons into the plasma.

It should be understood that the principles of the present invention are not restricted for use with the structures described hereinabove. For example, the concept of producing a line spectrum with the device of FIG. 3 could also be utilized with a thermonuclear-type device such as described in the above-mentioned Luce patent.

This invention has been described by way of illustration rather than by way of limitation and it should be apparent that it is equally applicable in fields other than those described.

What is claimed is:

1. In a Penning-type discharge device including a pair of parallel cathode electrodes, a hollow anode electrode spaced midway between said cathode electrodes and parallel thereto, an evacuated chamber enclosing said anode and cathode electrodes, means for providing a magnetic field with field lines oriented perpendicular to the planes of said anode and cathode electrodes, and a source of supply voltage connected "between said anode electrode and cathode electrodes to provide a refluxing discharge between said cathode electrodes, the improvement wherein one of said electrodes comprises two separated half-sections, one of said half-sections being connected to one side of said supply voltage, and wherein said half-sections are provided with an adjustable parallel resonant circuit connected therebetween, said circuit including means for tuning it to a frequency within the range 15 to 30 mHz to provide a harmonic line spectra of the frequencies in the plasma of said discharge, thereby substantially stabilizing said plasma.

2. The device set forth in claim 1, wherein said anode is in the shape of a ring, and said electrode having two separated half-sections is one of said cathode electrodes.

3. The device set forth in claim 2, wherein said mag- 6 netic field is of a selected value in the range from 1000 References Cited to 2500 gauss, the pressure within said chamber is about UNITED STATES PATENTS 10 torr, and the supply voltage is about 2500 volts.

4. The device set forth in claim 2, wherein a trans- 3,260,844 7/1966 P Y al M 328230X former is connected in the power supply lead to said 5 3316443 4/1967 Hansen 3137X anode, whereby an audiofrequency signal may be applied ALFRED L. BRODY Primary Examiner to said anode by way of said transformer to provide frequency modulation to the output signal from said US. Cl. X.R.

device. 10 3l37; 32433; 32823() 

